Homebrew PCBs

Hi,

I'm trying to print my first ever PCBs, but I'm having some problems with
the photoresist producing small bubbles (or patches lacking in
photoresist) which are about 2 mm (0.1") wide.

I'm using copper-coated blank PCB which has been polished with emery
paper, washed in clean water, and dried in a cool gas-fired oven (~ 10
minutes at ~ 100-150 degrees centigrade), in the very hot summer sun.

I'm then spraying on positive photoresist (Electrolube PRP200) from a can,
and leaving the boards to dry overnight in a dark cupboard, before
applying a photocopied acetate template of the tracks and exposing to
direct sunlight for about 5 minutes.

I'm etching using ammonium persulphate however the problems are apparent
before etching.

If I spray on a very thin layer of the photoresist then this forms very
small bubbles ( < 0.5 mm or 1/50 ") when etched, however this seems to be
too thin a layer to withstand etching, and the bubbles form pinhole voids
in the etched copper.

If I spray on about 2-3 times as much photoresist (producing a
significantly dark layer) then the larger bubbles form when the resist
dries .. although they are sometimes obvious immediately.


Any suggestions as to what causes this problem would be appreciated.

I think that the way in which I clean the boards should eliminate dirt and
moisture as causes, but I'm open to any ideas.

Thanks in advance

Scott Gordon
Brisbane, Australia

You don't spray the resist on the still hot board from the cool oven and 
the very hot summer sun?
(nice to tell me that while here i freeze off my ass if i go outside. ;-) )


Are the bubbles lifted off by gas pressure (inflated bubbles)
or are they just spots where there is no adhesion of the resist.


Don't the bubbles (the small ones) show up when developing?

how are the temperatures of etchant/developer?

i have no experience with spray on resist, maybe the manufacturer knows the 
problem?

Stefan
(On the cold side of the planet)

On Sun, 28 Dec 2003 08:47:51 +1000 (EST), Scott Gordon <scottG@...> 
wrote:

> Hi,
>
> I'm trying to print my first ever PCBs, but I'm having some problems with
> the photoresist producing small bubbles (or patches lacking in
> photoresist) which are about 2 mm (0.1") wide.
>

Here is how I prepare a board. All of the following is done while wearing
rubber gloves:
 
1.) With a new, clean paper towel, scrub the board with Comet or Ajax.
     Wash thouroghly with water and remove all cleaner residue. Dry the board
     with a paper towel.
2.) Rinse the board with acetone to remove any oil residue that may be on the 
    board (This is probably your problem from the description you gave). Dry
    the board.
3.) Apply photo resist.
 
If you air dry the board, you going to have some amount of dust that gets on the board. This can cause spots or bubbles. Therefore, I always dry the photoresist in a small oven to avoid this problem.
 
 
Best regards
 
 
Marvin Dickens
Alpharetta, Georgia  USA
Scott Gordon <scottG@...> wrote:
Hi,

I'm trying to print my first ever PCBs, but I'm having some problems with
the photoresist producing small bubbles 

Registered Linux User No. 80253
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[Non-text portions of this message have been removed]

Hi,

Thank you to those who replied to my post regarding bubble-like patterns
which appeared in photoresist on my PCBs.

The problem did indeed turn out to be boards not cleaned adequately
(apparently the resist manufacturer's instructions weren't robust enough).

One of the respondents turned out to be in my home town so I gave him a
call (thanks David!).

I'm now producing nice PCBs by cleaning the board with Ajax, heating it in
an oven to dry out the copper, and then being more careful with the
photoresist spray (after the board has cooled).

I still have to work out how to drill holes without breaking the drill
bits. I'm using a hand-powered drill and breaking one bit per 100 holes.

Thanks everyone,

Scott Gordon
Brisbane Australia.

Scott Gordon wrote:
> 
> I still have to work out how to drill holes without breaking the drill
> bits. I'm using a hand-powered drill and breaking one bit per 100 holes.
> 
   If these are small carbide bits, then that's actually pretty good by 
hand or even basic drill press, which will not give you fine enough 
vertical by hand for small bits.  The small jerks of your hand lowering 
the bit into uneven material will snap your bits like twigs.

   Make something simple that will give much finer vertical control and 
you should get a lot more holes.

Alan